boss up

In sheet lead work, bossing up is the process of folding and shaping lead at an internal or external corner to create a watertight seal without puncturing the metal. When two sheets of lead meet at an angle, simply creasing the corner leaves a vulnerable gap or potential hole at the apex. Bossing up compresses and forms the lead so it bridges that angle continuously, maintaining the integrity of the waterproofing layer.

The technique involves heating the lead slightly (typically with a lead welding iron or pallet) to make it malleable, then using wooden or leather mallets and dollies to gently hammer and fold the material over the corner. The lead is worked from both sides of the angle, gradually building up thickness and closing any voids. The result is a smooth, consolidated mass of lead at the corner that flows with the sheet rather than tearing or creating a pinched seam.

Bossing up is essential in roof valleys, parapet edges, and drip details where sheet lead must turn a corner while maintaining a continuous water barrier. Poor technique leaves stress points that crack under thermal movement or physical damage. Corners that are simply folded without working the lead can separate over time, particularly at pitches steeper than 45 degrees where gravity works against the seal.

The term comes from the resulting shape: a boss is a raised or thickened area, and the lead at a bossed corner sits proud of the surrounding sheet, formed into a solid knob or ridge. Plumbers distinguish this from soldering (which joins sheets with solder) and from simple folding (which leaves voids). Competent sheet lead work depends entirely on proper bossing; it is one of the first skills apprentices must master before working on high-risk details like flashings or tank construction.